Mechanism for tumor cell-induced immunosuppression and reversal of immunosuppression by macrophages: Some tumor cells are found to be directly suppressive for T cell-mediated immune response. After immune response has been initiated, tumor cells can recruit host macrophages to induce suppression. However, if peritoneal macrophages are present at the onset of the immune response, these macrophages can reverse the immuno-suppressive effect of tumor cells. On studying the mechanism for these phenomena, it has been revealed that suppression and reversal of suppression are critically dependent on the interactions among two populations of macrophages (splenic and peritoneal), T cells and tumor cells. The outcome may determine the fate of host immunity against tumor growth. Immunocompetence of tumor bearers: The effect of tumor cells on various populations of host lymphoid cells varies. The functions of some lymphocytes in tumor bearers are suppressed by the presence of tumor cells whereas other functions may be enhanced. Further, the net effect of the presence of tumor cells may depend on the interactions of various populations of lymphocytes.